How to determine file type from kernel module?

Question

Is there something like struct dirent* -> d_type that contains DT_REG, DT_DIR, DT_SOCK and etc. for kernel structures, for example for struct file? Looking at its fields, I cant find anything for this purpose.

Maybe someone knows how readdir determines d_type? I am looking at its implementation here https://github.com/lattera/glibc/blob/master/dirent/readdir.c and I cant understand what is going here.

Ubuntu18.04, 4.15.0-45 kernel version

Answer 1

The struct inode field i_mode is a bit-field that can be checked using the standard S_ISDIR, S_ISREG, S_ISLNK et al macros:

/*
 * Keep mostly read-only and often accessed (especially for
 * the RCU path lookup and 'stat' data) fields at the beginning
 * of the 'struct inode'
 */
struct inode {
    umode_t         i_mode;
    unsigned short      i_opflags;
    kuid_t          i_uid;
    kgid_t          i_gid;
       .
       .
       .

An example of its use in ext4 kernel code:

/*
 * Test whether an inode is a fast symlink.
 * A fast symlink has its symlink data stored in ext4_inode_info->i_data.
 */
int ext4_inode_is_fast_symlink(struct inode *inode)
{
    if (!(EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL)) {
        int ea_blocks = EXT4_I(inode)->i_file_acl ?
                EXT4_CLUSTER_SIZE(inode->i_sb) >> 9 : 0;

        if (ext4_has_inline_data(inode))
            return 0;

        return (S_ISLNK(inode->i_mode) && inode->i_blocks - ea_blocks == 0);
    }
    return S_ISLNK(inode->i_mode) && inode->i_size &&
           (inode->i_size < EXT4_N_BLOCKS * 4);
}

Note that you need to be really careful traversing such kernel structures. If you don't take the proper locks, they can change out from under the thread examining them.